Type JN1, a nitrogen-strengthened austenitic stainless steel [Fe-24Cr-15Ni-4Mn-0.32N (wt%)], is emerging as the preferred structural material for superconducting fusion magnet casings to operate in liquid helium at 4 K. In order to evaluate the fracture toughness and the adiabatic heating of thick-section (200 mm) weldment in JN1 forged stainless-steel plate with Ni-base filler metals, elastic-plastic fracture toughness tests were performed with 25-mm-thick compact tension specimens at 4 K. Thermocouples were used to measure the temperature rise near the crack tip. The effect of specimen location and nitrogen content on the fracture toughness parameters, and the temperature rise during dynamic crack growth, are discussed.